How can i select the half of the surface of the cylinder?

I`m designing the peristaltic pump for my project. I want to know how much normal force would be exerted on the roller when the roller squeezes the tube.

To find the force value, i tried static simulation on the surface of the tube and selected the surface of the tube, then Creo just selected the all the surfaces of the tube Not the half of the surface that i want to put the force on. Could you tell me how to select the half of the cylinder or sphere surface?